In many industries, among them electricity generating stations, there is a need to isolate a liquid, stored in liquid storage tanks, from the atmospheric oxygen. An example of this is the storage of demineralized oxygen-free condensate. This condensate must be kept from contact with the atmospheric oxygen, as oxygenated condensate is very corrosive to the piping and equipment through which it flows.
A common practice is to blanket these liquid storage tanks with a low pressure (approximately half inch of water column) blanketing gas, such as nitrogen. Many such tanks are used in installations where the tanks' operability must be preserved under adverse circumstances and, thus, the tanks' pressure relieving devices must be designed for a missile impact. Also, the mechanical failure of these pressure relieving devices must be considered. These requirements lead to very expensive design solutions. Such solutions are, for example, designing the tank to a higher pressure than would otherwise be required, equipping the tank with redundant pressure relieving devices, and protecting those pressure relieving devices with expensive missile barriers.
Additionally, in many cases, a conversion of an atmospheric liquid storage tank to a gas-blanketed tank is desired, combined with the need to preserve the operability of such a tank under adverse circumstances, such as a missile impact and equipment failure. A pressure relieving device for such a tank must have a minimal pressure loss when relieving vacuum in the tank and must not, by its weight and reaction forces, overload the roof of the existing liquid storage tank. Additionally, a minimum of maintenance is desired in most modern plants.
The need of an inexpensive solution of venting gas-blanketed safety-related tanks led to an innovative liquid seal vent design suitable for safety-related installations, designed for missile impact, with a feature of supportability by an external hanger, automatic replenishment of liquid in the liquid seal vent and in the tank's overflow pipe liquid seal, a minimum magnitude of the potential vacuum in the tank, and easy installation on an existing liquid storage tank without cutting the tank's roof or welding to the roof.
Although liquid seal vents were introduced in other previous patents, none of the previous designs are suitable for safety-related gas-blanketed liquid storage tanks, as those vents are not designed to assure that they would function after they are hit by an external missile. Additionally, none of the previous patents considered the feature of upholding the weight of the liquid seal vent by an external hanger, as the weight of the liquid seal vent can exceed the allowable weight on the tank's roof.
Also, none of the previous patents offered a design which automatically replenishes the liquid in the vent to accommodate liquid evaporation. The feature of automatic replenishment of liquid in the tank's overflow pipe liquid seal, by means of the roof liquid seal vent, was also not a part of the previous patents. Additionally, none of the previous patents addressed the need of adding such a vent on an already constructed tank. As many liquid storage tanks are lined internally with special coatings, the installation of the vent by cutting into the tank's roof and welding on the roof is expensive, as during such processes the internal lining is damaged.
The need for minimization of the vacuum that may develop inside the tank has also not been specifically addressed in previous patents, although most tanks have typically much lower allowable pressure in vacuum than in the overpressure.